cutting them like that give the maximum additional surface area, and that’s what increases cooling. Getting as much laminar flow through the grooves as possible will carry away the most heat. Since they’re right in the oncoming air when the bike is moving, and “fan effect” from radial grooves wouldn’t be a plus.

Architect, I have one of those on my panhead, they are rather rare, don’t know if it was a factory part or aftermarket. I have found that most people don’t know how to set up the front brake properly and say they don’t work well. They work pretty good when done right, not as good as a disc though.